\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{circuitikz}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes}
\ctikzset{bipoles/length=1cm}
\begin{document}
\small
\begin{circuitikz}[>=latex, scale=1.0,european]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
  \draw(0,0)--(5,0) to  (5,3);
  \draw (0,3) to [vR=$R$] (5,3);
  \draw[<->](0,.1) to node [left]{$U$} (0,3-.1);
  \draw (5,1.5) node[elmech]{};
  \draw [fill=white](0,0) circle (2pt);
  \draw [fill=white](0,3) circle (2pt);  
  \node at (5.7, 1.5){$r_0$};
\end{circuitikz}
\end{document}